Study On The Preparation And Performance Of Cathode Material Lithium Manganese Oxide For Power Lithium Ion Batteries

Power lithium ion battery is a hot research topic in the field of new energy,for it is the most important component of the electric vehicle.Cathode material is a key material,which is the main factor that influences the performances of lithium ion battery.Spinel lithium manganese oxide(LMO)is one of the most promising cathode materials for power lithium ion batteries.Some progress has been made in application research,but there are still some issues restricting the application of spinel LMO in large scale.The synthesis method of LMO should be perfected,and its energy density,cycle life,rate capability and security need to be further improved.In this paper,a new synthesis method called 'ion permeation high temperature reaction'(Ion permeation method)was developed.The influence of the crystal structure and morphology of the raw material manganese oxide on the performances of LMO was studied.Then,the modification for LMO was carried out using composite doping technology by Li-rich,niobium and phosphate polyanion and surface coating with solid electrolyte.Finally,the effect of the synthesis methoed and modification technology on the performances of the actual power lithium ion batteries was investigated.Manganese oxide reacts with lithium hydroxide monohydrate in the mixed organic polyol in the ion permeation method.Lithium hydroxide is dehydrated and dissolves with the help of organic polyol.Lithium ions permeate into the tunnel structure of manganese oxide under the action of solution heat and combinate with manganese oxide matrix at a molecular level.In the progress of high temperature reaction,the spinel LMO with high degree of crystallization and pure phase can be obtained sintering only at 800? by ion permeation method,while the spinel LMO with slightly high degree of crystallization can be obtained when sintering at above 900? in traditional solid state method.In the same synthesis conditions,the capacity and the cycle performance of the sample prepared by ion permeation method are superior to that of the sample prepared by solid state method.' The product prepared by ion permeation method has the physical properties of high density and small specific surface area.When using the ion permeation method,the crystal structure of manganese oxide has vital effect on the final electrochemical performance of LMO.?-MnO2 and R-MnO2 possess(2x2)square tunnel structure and(2x1)rectangular tunnel structure which are formed according to the different arrangement of the basic structural unit of MnO6 octahedron.Lithium ions easily diffuse into the tunnels of?-MnO2 and R-MnO2 during the process of preparing precursor.?-MnO2 and Mn3O4 possess narrow(lxl)square tunnel and triangular tunnel,while Mn2O3 does not contain tunnel,so lithium ions can not enter the lattice of these manganese oxide.The capacity and the cycle performance of the sample prepared by ?-MnO2 and R-MnO2 are superior to the LMO prepared by ?-MnO2,Mn2O3 and Mn3O4.The influence of the morphology of MnO2 on the performance of LMO was studied also.The LMO prepared using spherical nanorod-MnO2 aggregation shows the highest capacity and the best cycle performance.The spherical shape helps to restrain the manganese from dissolving,so it improves the cycling performance at high temperature of 55?.In order to further enhance the electrochemical performance,the niobium cation,lithium cation and phosphate anion were doped simultaneously in LMO.After doping niobium,the result of XRD show that the crystal of LMO has a tendency to preferred orientation along the(311)and(400)surface.The primary crystalline grains transform into sphere shape from octahedral shape.The unit cell parameter is increased and the Mn-O bond is strengthened.In addition,the average valence of manganese is reduced after doping niobium.The change of crystal structure and the valence of Mn improve the capacity,cycle life and rate capability of Li-rich LMO.It solves the problem that it must decrease the capacity in order to improve cycle stability usually.Doping phosphate further enhances the stability of LMO.Comparing with Li1-1Mn1.9O4,the thermal decomposition temperature of Li1.1Mn1.87Nb0.03O4 and Li1.1Mn1.87Nb0.03O3.98(PO4)0.02 rises to 346? and 361? from 330? in state of charge.An additional surface modification of Li1.1Mn1.87Nb0.03O3.98(PO4)0.02 was studied with a solid electrolyte compound Li1.4Al0.4Ti1.6(PO4)3(LATP)which has never been used as coating agent by sol-gel method.Coating LATP significantly improves the cycling performance.The discharge capacity retention ratio of the sample Li1.1Mn1.87Nb0.03O3.98(PO4)0.02 coated with an amount of 0.5 wt%LATP reaches 99%at 1 C rate after 100 cycles at 25? and 96.8%at 55?.Comparing with the treatment of TiO2,LATP can promote the diffusion of Li-ion and reduce the ion transfer impedance in the electrode surface,so it enhances the rate capability of LMO,while TiO2 increases ion transfer impedance and reduces the rate capability of LMO.At 20 C rates,the LMO coated with an amount of 0.5 wt%LATP can deliver 81.3%of the total capacity,while the sample coated with an amount of 0.5 wt%TiO2 only deliver 74.5%of the total capacity.In the last section,the research results of a actual power battery using the modified graphite as anode show that the weight specific energy density of the battery using the Li-rich LMO doped with niobium and phosphate simultaneously as cathode increases by 10%comparaed with that of the battery using the Li-rich LMO,and the volumetric energy density increases by 15%.Doping also improves the cycling stability,rate performance and discharge capability in low temperature of the power battery.Coating LATP further enhances the cycling stability at high temperature(55?)and the resistance capability to overcharge(3 C/10 V).The specific weight energy density of the 20 Ah power battery using spherical Li1.1Mn1.87Nb0.03O3.98(PO4)0.02·LATP as cathode material reaches 155 W h kg-1,the capacity retention ratio reaches 95.2%at room temperature(25?)after 500 cycles and 94.8%at high temperature(55?)after 100 cycles.93.1%of the total capacity can be attained at 6 C rates,90.5%of the total capacity can be attained at-20?,at the same time it can retain high discharge voltage plateau.The experimental data proves that the LMO prepared in this work can satisfy the requirements of the power batteries.